Process for the formation of fibrous webs of staple fiber from continuous textile filaments

ABSTRACT

Continuous textile filamentary material is converted into a web of short fibers of graduated length by shattering the filaments in an air jet stream of a velocity which is at least 1.5 times sonic velocity, at a calculated temperature of not over -100*F.; diffusing the resultant short fibers into a plenum chamber; and collecting the fibers in web form on a conveyor.

United States Patent [1 1 Marshall Aug. 14, 1973 PROCESS FOR THE FORMATION OF FIBROUS WEBS OF STAPLE FIBER FROM CONTINUOUS TEXTILE FILAMENTS [75] lnventor: Preston F. Marshall, Walpole, Mass.

[73] Assignee: The Kendall Company, Walpole,

" Mass.

221 Filed: Jan. 28, 1971 211 Appl.No.: 110,446

UNITED STATES PATENTS 2,728,699 12/1955 Labino 161/169 2,956,717 10/1960 Scharf 225/1 3,016,599 l/l962 Perry 161/169 3,379,001 4/1968 Campbell et al 161/169 Primary Examiner-Morris Sussman Attorney-John F. Ryan [5 7] ABSTRACT Continuous textile filamentary material is converted into a web of short fibers of graduated length by shattering the filaments in an air jet stream of a velocity which is at least 1.5 times sonic velocity, at a calculated temperature of not over 100F.; diffusing the resultant short fibers into a plenum chamber; and collecting the fibers in web form on a conveyor.

2/1945 Magill 161/169 CONTINUOUS FILAMENT TOW HIGH VELOCITY CRYOGENIC AIR STREAM FROM LAVAL NOZZLE HIGH VELOCITY AIR STREAM OF SHORT FIBERS PLENUM CHAMBER DEACCELERATED FIBERS AT ROOM TEMPERATURE CONTINUOUS COLLECTION DEVICE FI BROUS WEB 1 Claim, 5 Drawing Figures CONTINUOUS FILAMENT TOW HIGH VELOCITY CRYOGENIC AIR STREAM FROM LAVAL NOZZLE HIGH VELOCITY AIR STREAM OF SHORT FIBERS PLENUM CHAMBER DEACCELERATED FIBERS AT ROOM TEMPERATURE CONTINUOUS COLLECTION DEVICE Fl BROUS WEB IFIG. 1

PAIENTED AUG 1 4 SHEEI 2 BF 3 IFIG.

PROCESS FOR THE FORMATION OF FIBROUS WEBS OF STAPLE FIBER FROM CONTINUOUS TEXTILE FILAMENTS This invention relates to the pneumatic cutting of strands of textile filamentary material into staple fibers of shorter lengths, and to the collection of the shorterlength fibers into a uniform web or fleece. More particularly it relates to such an operation wherein the filamentary material is subjected to the action of a highvelocity air stream at sub-zero temperatures, thereby breaking the filaments into shorter-length fibers which are dispersed in substantially discrete form in the expanding air stream, from which they are removed in web form.

In the formation of fibrouswebs to be fashioned into nonwoven fabrics, batting, coil, and the like, it is customary to feed a supply of staple fibers to a card, garnett, air-lay machine, or the like, which separates discrete and small groups or clumps of fibers from the feed mass and assembles these discrete clumps in the form of a fibrous web.

There are at least two disadvantages in the production of such fibrous webs by conventional methods. First, man-made fibers are produced in the form of a bundle or tow of continuous filaments, which must be cut to staple length by a tow cutter, Pacific Converter, or the like. Second, and more importantly, the conventional web-forming devices mentioned above do not separate fibers completely from each other, but instead they pluck off small groups or clumps of fibers from the main fibrous mass, so that card webs and garnett webs have a blotchy appearance. Furthermore, a fibrous web composed of fibrous clumps or aggregates does not develop the tensile strength which it would possess if the fibrous elements were substantially completely separated from one another and then reassembled in web form.

lt has now been found that if a tow of, for example, continuous viscose rayon or acetate filaments is fed at a controlled rate into an air stream passing through a Laval jet under such conditions that the velocity of the air stream is at least 1.5 times sonic velocity, and the air temperature in the expanding or divergent part of the jet has a calculated dropto -lF. or lower, then the filaments are shattered into individual fiber of controllable length, and these fibers are dispersed in the air stream as individual fibers free from each other. The fiber thus produced are exhausted into a plenum chamber to dissipate their velocity, whence they are collected continuously in the form of a web or fleece.

The process of this invention thereby utilizes cryogenic temperatures to increase the density of the impacting fluid, and by increasing the velocity of the fluid to at least 1.5 sonic, the' necessary increase in forceimpact on the filaments is realized. This combination of low temperature and high velocity has proved effective in shattering filaments where low density (noncryogenic) jet streams of the same Mach number are ineffective. Heretofore it has been assumed that since the effect of compressibility of a fluid upon its motion is determined primarily by its Mach number, that, therefore, only the Mach number determined the action of a gas stream on a filament to be shattered. The present invention utilizes the hitherto unappreciated increase in the impact shattering of a fiber by forces derived from fcold gas impact as opposed to hot gas impact." Even where the hot gas impact is has the same Mach number as a cold gas impact, it is less effective in filament shattering.

The exposure of filamentary strands to pneumatic forces is a well-known art, commercially employed in the texturizing of continuous-filament yarns to increase their bulk by the formation, in a turbulent air stream, of stable or metastable loops, kinks, and filamentary entanglements. In such processes it is known that excessive air pressures, above certain critical limits, will shatter some of the filaments if said filaments have a low flex life, as explained in U. S. Pat. No. 2,869,967. The shattering of some filaments is sometimes desirable, to give a' yarn with protruding filament ends. However, in order to avoid excessive filament breakage, such yarn-texturing devices are customarily operated at about sonic velocity.

It is also known to break inorganic filaments, such as glass, into fibers by impinging an air jet onto a bundle or tow of such filaments, and to collect the resulting fibers in a batt, as described in U. S. Pat. No. 1,938,982. However, such processes are in general inoperative if so-called textile fibers (viscose, acetate, acrylics, nylons, polyesters) are used as feed stock instead of glass or mineral fibers.

The aerodynamic shattering of long filaments into short fibers is due to a complex of effects, and has been found to be related to, among other things, the flex life of the filaments at the temperature of the air stream being employed. It is recognized that glass filaments have an insignificant flex life: a test involving the flexing of filamentary bundles rated glass at l, acetate at 300, viscose at 3,000, and nylon and polyester at 1,000,000. While flexing is probably only one factor in the process of this invention, the flex life of the filaments involved is a useful index in determining the jet velocities and temperatures to be employed in producing the webs of this invention.

The process of the present invention makes possible the'useof a web-forming device that is less expensive, smaller, lighter, and less demanding of maintenance than conventional web-forming devices. It is also capable of forming webs of fibers which, because of unusually low denier, or because of lack of crimp, or finish, are not processable by such conventional devices. Additionally, since the process of this invention results in an air-borne fiber dispersion in which substantially all fibers are separated from one another, the resulting fiber webs display a uniformity of density and freedom from mottled or blotchy appearance which cannot be realized with conventional webforming techniques.

It is an object of this invention to provide a process for the simultaneous aerodynamic cutting and dispersing of filamentary textile material into individuallydispersed short-fiber form.

[t is an additional object of the invention to provide a simple web-fonning device capable of producing fibrous webs of improved uniformity of density.

The invention will be better understood with reference to the following description and drawings, in which H0. 1 is a flow-sheet of the process of this invention.

FIG. 2 is a side view of an apparatus suitable for the practice of the invention.

FIG. 3 is a front elevation of the plenum chamber of FIG. 2.

FIG. 4 is a cross-sectional view of a nozzle suitable for use in the invention.

FIG. 5 is an enlarged cross-sectional view, partly broken away, of the throat section of the nozzle of FIG. 4.

Referring to FIG. 1, a bundle or tow of continuous filaments of textile filamentary material is fed at a controlled rate to a convergent-divergent (Laval) nozzle, said nozzle being powered by air pressure. The air pressure and the nozzle dimensions are so chosen as to create a jet stream in which the calculated temperature of the air is at least 100F. and the stream velocity is at least 1.5 times sonic velocity. A jet stream of such characteristics, as is known, will devolve into shock waves which can be shown photographically. Under these conditions, it has been found that even filaments which normally possess a substantial flex life are completely shattered into short fibers, which are substantially freed from contact or entanglement with each other in the plenum chamber, which constitutes an expansion or decelerating mechanism in which the temperature of the air stream returns substantially to room temperature, and in which the fibers are converted into a uniform dispersion in the air stream. The air-borne stream of individual fibers is then collected into a fibrous web by conventional devices such as a foraminous conveyor belt or revolving screen cylinder, optionally with the aid of a vacuum device to assist in the removal of excess air.

LAVAL NOZZLE FIG. 4 shows a filament-forwarding convergentdivergent nozzle suitable for the conversion of long textile filaments into short fibers, according to the process of this invention. By long textile filaments is meant those which are either continuous, as in filamentary tow or continuous filament yarn, or yarns or ropes of long fibers such as hemp, jute, or wool, which are so tightly aggregated by twisting that the feed rate of the material to the jet can be controlled i.e., so that the filamentary material is always under restraint from the feed rolls.

In its basic form the nozzle comprises a cylindrical chamber 12 capped at one end by an inlet cap 20 containing a tapered inlet 18 for the introduction of filamentary material, to which is affixed the filament guiding tube 16, located substantially along the central axis of the device. The other end of the chamber 12 is capped by an exit cap 24, which restrains the straight exit section 22 and the convergent-divergent nozzle section 23 of the device. For convenience in machining, sections 22 and 23 are separate pieces, fitting in sliding relationship in the inner chamber 15, which in operation are held tightly against the cap 24 by air pressure.

Air under pressure is fed to the inner chamber by means of the threaded air connection 14. The distance to which the filament guide 16 projects past the nozzle portion 23 of the device is adjustable by means of the threaded cap 20.

In order to obtain a desired velocity in an air stream issuing from a convergent-divergent nozzle, a critical ratio of absolute exit pressure to absolute air pressure delivered to the nozzle must be attained. This, in turn, involves a certain ratio of area of the exit portion of the device to the net throat area of the nozzle, as explained more fully below. The ratios of exit or back pressure (assumed to be substantially atmospheric) to the pressure applied ahead of the nozzle is calculated from the isentropic gas dynamic laws, and such engineering data is set forth, for example, in Marks Mechanical Engineering Handbook, Fifth Edition, page 1491:

Velocity in Applied Pressure Mach Number Back Pressure 1.00 .528

In operation, air at to 500 pounds P.S.l.G. is fed to the air inlet 14, and is converted to a convergent stream of air in the convergent section 21 of the nozzle, whence it diverges in the divergent section 25. A bundle of filaments is fed to the opening 18, and is drawn through the tubular guide 16 to be expelled through the nozzle, into the divergent section 25, and leaves the device through the exit orifice 26 in the cap 24.

The dimensions and relative proportions of a suitable convergent-divergent nozzle will of course vary with the denier of the filamentary tow used as a source of fibers, and with the output poundage at which it is desired to operate the process. The device of FIG. 4 may, for example, be formed from a 5 inch length of brass pipe 1 inch [.D., with a similar piece of pipe 14 silversoldered thereto. When using a light filamentary tow (4,400 denier viscose rayon, 2,000 filaments), the yarn opening 18 may taper down to a guiding tube diameter of 0.100 inches, the nozzle being 0.300 inches in diameter at the throat section, expanding to 0.440 inches in diameter in the exit section.

The employment of filamentary tow of greater denier, for higher poundage output, will necessitate larger filament input tubes and larger nozzles: however, the scaling-up of the apparatus is a matter of engineering detail, the important factor being that the ratio of throat area to divergent area be properly chosen so that a Mach number of at least 1.5 be attained in the air stream, the divergence depending on the air pressure.

The exemplary throat diameter of 0.300 inches for a light filamentary tow, given above, is a gross diameter: that is, the effective throat diameter so far as the air stream is concerned is not the distance B of FIG. 5, but B reduced by the outside diameter of the filament inlet tube 16. The ratio of exit area to net throat area, as illustrated in FIG. 5, becomes area A: (area 8- area C).

The following types of nozzles have been found suitable for the conversion of filamentary material into a stream of short fibers:

NOZZLE TYPE A B C D Throat Diameter 0.424 in. 0.624 in. 1.120 in. 1.120 in. Angle of Divergent Taper 5.7 5.7" 5.7" 5.7 Exit Section Diameter 0.500 in. 0.704 in. 1.200 in. 1.120 in. Exit Section Length 3.25 in. 3.00 in. 5.00 in. 5.00 in. CD. of lnlet Tube 0.300 in. 0.500 in. 1.000 in. 1.000 in. Net Throat Area 0.0707 sq. 0.110 sq. 0.200 sq. 0.200 sq. in. in. in. in.

Types C and D, with filament inlet tube diameters of one inch, are suitable for processing heavy tow of up to 500,000 denier.

FlLAMENT BREAKAGE As stated above, in the process of this invention a shock zone is created by allowing air at high pressure and room temperature to expand isentropically to atmospheric pressure, with a consequent drop in temperature and the release of kinetic energy.

For the formation of a stream or plasma of individually separated short fibers, a certain residence time must be established for the filaments in the shock zone. The process may be viewed as a residence timedependent one, in which the time required to fragment all of the filaments is proportional to the mass of filaments to be altered, and inversely proportional to the energy which they absorb.

In what may be regarded as normal operation, the filaments emerging from the orifice 26 are visibly continuous for as much as 6 inches beyond the orifice, at about which point they are transformed into a stream of individualized fibers. With a constant rate of filamentary feed, raising the air pressure causes the shattering effect to take place closer and closer to the orifice 26, or even within the exit chamber 25.

DECELERATION AND DIFFUSION In order to decelerate the high-speed stream of short fibers to a manageable velocity, it has been found convenient to employ a plenum chamber, into which the high-speed fibrous stream is exhausted. Such a device is shown in FIGS. 2 and 3, in side and front elevation respectively.

Referring to FIG. 2, there is shown a suitable plenum chamber with the side panel removed. A filamentary tow, 11, is fed at a controlled rate by the feed rolls, 17, 17, to the nozzle 10 containing the jet which creates an air stream of at least Mach 1.5 The resulting high speed fibrous stream, as it issues from the jet device 10, diffuses a stream of individually separated short fibers into the upper chamber 30. Although various types of plenum chamber may be used, for convenience and economy of space the device as shown is divided into an upper, middle, and lower chamber 30, 32, and 34 respectively, separated by the plates 42, 44, and 46 which extend the full width of the chamber. Plates 42 and 44 do not extend the full length of the chamber, however, so that the path of the decelerating fibers is a serpentine one, as shown by the dotted line. The rear and front partitions, 36 and 38, act as baffles, deflecting the air stream, so that a constant and substantially uniform flow of fibers, at a manageable velocity, emerges from the plenum chamber exit 48 to impinge on the upper surface of the screen conveyor belt 50. If desired, a conventional vacuum box 52 may be mounted on the under side of the porous conveyor to bleed off the last traces of air and assure proper deposition of the fibrous web 54 on the conveyor.

In some cases, where the web is to be subjected to subsequent treatment such as bonding or impregnation, it may be found that the short-fibered web has insufficient strength to survive the rigors of wet processing. In such instances it is convenient to interpose between the conveyor screen and the fibrous stream a layer of permeable supportive material such as gauze, cellulose tissue, porous nonwoven fabric, or the like. Such expedients are well-known in the art, and are not shown.

The term manageable velocity" as employed above means a stream velocity at which the fibers can be deposited continuously onto a moving porous belt with substantial absence of fiber clumping or deflection of the stream. The purpose of the plenum chamber or diffusor, therefore, is to spread the high speed jet stream over a large cross section, so that the kinetic energy of the stream is transferred to pressure, by diffusion. This pressure forces the air through the porous conveyor, which filters out the fibers in web or fleece form. A convenient range of exit velocity that is, the air velocity at which the decelerated fibrous stream impinges on the porous conveyor belt has been found to be 3 to 30 feet per second. 1

At the divergent section of the jet, the air velocity is at least Mach 1.5, or 1400 feet per second at F. Throttling this velocity down to a managable exit velocity is a function of the parameters of the plenum chamber, which can be calculated from a consideration of the volume of air to be handled.

The invention will be illustrated by the following examples. In each case the filamentary tow was fed to the jet at a rate controlled by the feed rolls 17, 17, and a three-section plenum chamber was used as in FIGS. 2 and 3. The upper chamber 30 was 20 inches square in cross-section, with a circular opening 40 of 16 inch diameter. The middle chamber 32 was 30 inches wide and 6 inches deep, while the bottom chamber was 40 inches wide and 4.5 inches deep. The length of the plenum chamber was 40 inches.

EXAMPLE 1.

A 4400 denier 2934 filament rayon tow was fed at a rate of feet per minute or 2.53 pounds per hour to a type B jet powdered by air at 125 PSIG. The resulting fibrous stream was deposited as a 40 inch wide shortfibered web on the conveyor belt moving at 2.57 feet per minute, the web weighing 18.7 grams per square yard.

EXAMPLE 2.

The same rayon tow as in Example 1 was fed at a rate of 187.5 feet per minute or 3.71 pounds per hour to a type A jet powered by air at 210 PSIG. The resulting 40 inch web weighed 20.4 grams per square yard when deposited on the conveyor belt at a rate of 5.1 feet per minute.

EXAMPLE.- 3.

A 900 denier 128 filament acetate tow was fed at 187.5 feet per minute or 6.1 pounds per hour to a type A jet powered by air at 220 PSIG. The resulting 40 inch web weighed 16.0 grams per square yard when deposited on the conveyor belt at a speed of 7.7 feet per minute.

EXAMPLE 4.

The same rayon tow as in examples 1 and 2 was fed at a rate of 375 feet per minute or 7.4 pounds per hour to a Type A jet powdered by air at 230 PSIG. The resulting 40 inch web weighed 19.6 grams per square yard when deposited on the conveyor belt at a speed of 7.7 feet per minute.

EXAMPLE 5.

The same acetate tow as in example 3 was fed at a rate of 375 feet per minute or 12.2 pounds per hour to a type A jet powered by air at 235 PSIG. The resulting 40 inch web weighed 32.0 grams per square yard when deposited on the conveyor belt at a speed of 7.7 feet per minute.

EXAMPLE 6.

A 250,000 denier 833,333 filament black rayon tow was fed at a rate of 17.3 feet per minute or 19.4 pounds per hour to a type B jet, powered by air at 235 PSIG. The resulting 40 inch wide web weighed 68.3 grams per square yard when deposited on the conveyor belt at a speed of 5.8 feet per minute.

EXAMPLE 7.

A jute twine, 4 ply S twist, of approximately 44,640 denier was fed at a rate of 23.4 feet per minute or 4.7 pounds per hour to a type B jet, powdered by air at 225 PSlG. The resulting web weighed 16.5 grams per square yard and was produced at a rate of 5.8 feet per minutes. It was exceptionally lofty, due presumably to the resilient nature of the short bast fibers, which were randomly distributed in three dimensions throughout the web.

Fibrous webs can be produced by the process of this invention which are of remarkably uniform density and opacity, and substantially free from the streaks and light spots characteristic of carded or garnetted webs. This is due to the unique nature of the process, in that each fiber is carried in the air stream as a single fiber, and the fibers are not clumped and entangled into irregularly shaped clusters as they are in a mechanical carding action. In part also this uniformity is the result of the fact that the web produced by the process of this invention comprises fibers which vary widely in length. The aerodynamic shattering oflong filaments into short fibers, unlike tow-cutting by mechanical means, is a random event, in that the filaments are broken into fibers as the result of non-uniform shock waves, the effect of which is complicated by the non-uniform resistance to breaking caused by minute and perhaps microscopic variations in structure along the length of the filaments.

A fiber length analysis was carried out on the web of Example 3 by hand-drawing a sample of fibers according to ASTM Test 2142, laying out the fibers on a plush covered board. The sample was separated into three groups of descending fiber lengths, and each group was weighed and the longest fiber in each group was measured.

From this it was determined that the longest fiber in the array was 3.1 inches; that 75 percent of the total weight was made up of fibers less than 1.1 inches in length; and that 28 percent of the weight of the fibers were 0.5 inches or less in length. By plotting these re sults on probability paper, the probable average length of the fibers was estimated at 0.75 inches.

The resistance to shattering of a filament is a function of the chemical nature and physical history (orientation, crystallinity) of the fiber, and of its thickness or denier. Thus, when the 1.5 denier viscose fibers of the web of Example 1 were analyzed, practically all of the fibers were found to be less than 0.5 inches in length, ranging down to lengths too short to be measured by conventional methods.

The effect of combining fibers of different lengths into a homogeneously intermingled array is to provide a network of longer stress-bearing fibers into which the shorter fibers infiltrate, so that the voids and air cells in the long-fibered network are minimized. In this way, a pleasing degree of uniformity is realized which cannot be readily obtained by other methods. ln mechanical carding, short fibers are rejected. ln papermaking techniques, it is difficult to incorporate textile-length fibers, especially fibers of varying lengths, without expensive modifications of conventional papermaking techniques. 1n the webs of my invention, fibers of a variety of lengths are uniformly distributed throughout the length, breadth, and thickness of the web.

It is characteristic of the products of this invention that the fibers vary more or less gradually in length from the longest to the shortest, with at least 50 percent of the fibers being not over one-half as long as the longest fibers, and at least 25 percent of the fibers being not over one-quarter as long as the longest fibers. Such a graduated degree of distribution in fibrous webs is not readily realized by any other process of which 1 am aware.

Depending on the web strength and the desired end use, the webs of the process of this invention may be converted directly into nonwoven fabrics by impregnating or spraying with a bonding material, or they may be combined with paper, films or plastic nets, knitted or woven fabrics, and the like.

Having thus described my invention, 1 claim:

1. A process for the formation of fibrous webs of staple fiber from continuous textile filaments which comprises feeding said continuous textile filaments at a controlled rate into a cryogenic fluid stream,

said fluid stream having a velocity of at least Mach 1.5 and a calculated fluid temperature of less than F. to shatter the filaments into short fibers; decelerating and diffusing said fluid stream with the fibers therein to cause said fibers to slow down and spread out,

and collecting said short fibers into a web. 

1. A process for the formation of fibrous webs of staple fiber from continuous textile filaments which comprises feeding said continuous textile filaments at a controlled rate into a cryogenic fluid stream, said fluid stream having a velocity of at least Mach 1.5 and a calculated fluid temperature of less than -100*F. to shatter the filaments into short fibers; decelerating and diffusing said fluid stream with the fibers therein to cause said fibers to slow down and spread out, and collecting said short fibers into a web. 